Solvent-Free Hydrosilylation of Alkenes Catalyzed by Well-Defined Low-Valent Cobalt Catalysts

B(C6F5)3-Regulated Selectivity of Aryl Alkene Hydrosilylation Catalyzed by a [P,C]-Chelate Cobalt(I) Complex

In this article, two cobalt complexes bearing bidentate ligands, [Si,C]-chelate cobalt(I) complex [(Si,C)Co(PMe3)3] (1) and [P,C]-chelate cobalt(I) complex [(P,C)Co(PMe3)3] (2) were synthesized by activating Csp2-H of the corresponding 2-(diphenylsilylenoaminomethyl) pyridine (L1) PyN(Me)SiL (L = PhC(NtBu)2) or 2-(diphenylphosphinoaminomethyl) pyridine (L2) PyN(Me)PPh2 with CoMe(PMe3)4. The catalytic performance of complexes 1 and 2 for alkene hydrosilylation was studied. Because of the different electronic properties of the phosphine and the silylene pincer ligand, the catalytic effect of phosphine complex 2 is superior to that of silylene complex 1 as indicated by faster conversion and higher selectivity for most of the selected substrates. Both aromatic alkenes and alkyl alkenes are mainly anti-Markovnikov converted to products. In the study of the catalytic mechanism, cobalt(III) hydride A' is proposed as the key intermediate. Unexpectedly, when B(C6F5)3 is added to the system, the selectivity of the catalytic system for aromatic alkenes is reversed to afford mainly Markovnikov products. Through experimental exploration, it is proposed that the addition of B(C6F5)3 induces a coordination vacancy at the Co center, which favors the initial coordination of the olefin and, in turn, changes the catalytic reaction mechanism, resulting in a reversal of regioselectivity.

Synthesis of Dinuclear Cobalt Silylene Complexes and Their Catalytic Activity for Alkene Hydrosilylation Reactions

A novel dinuclear silylene cobalt complex [((Me3P)2Co)(PMe2)(CoCl(PMe3))(Si(NCH2PPh2)2C6H4)] (1) supported by the [PSi(silylene)P] ligand was prepared through the reaction of N-heterocyclic [PSiP] pincer ligand L1 (HSiCl(NCH2PPh2)2C6H4) with Co(PMe3)4. Complex [((Me3P)2Co)2(Si(NCH2PPh2)2C6H4)] (2) was formed through the reaction of complex 1 with MeLi. To the best of our knowledge, complexes 1 and 2 are the first examples of dinuclear silylene cobalt complexes supported by the [PSi(silylene)P] ligand. A new preligand L2 (SiCl2(NCH2PPh2)2C6H4) was synthesized, and the reaction of preligand L2 with Co(PMe3)4 afforded silyl cobalt complex [((Me3P)2Co)(SiCl(NCH2PPh2)2C6H4)] (3). The reaction of 3 with CO delivered cobalt carbonyl complex [((Me3P)(CO)Co)(Si(NCH2PPh2)2C6H4)]2O (4). The catalytic activity of cobalt complexes 1-4 on the hydrosilylation of alkenes was explored. Among the four complexes, complex 1 has the best catalytic activity. The catalytic process could be promoted with NaBHEt3 as an additive, and a complete conversion with an excellent selectivity of 98:2 (b/l) could be reached at 120 °C within 8 min for aryl alkenes. A possible catalytic cycle was proposed on the basis of the experimental results and literature reports, with a cobalt hydride complex as an active intermediate. The molecular structure of complexes 1-4 was determined by single-crystal X-ray diffraction analysis.

Catalytic Properties of [PSiP] Pincer Cobalt(II) Chlorides Supported by Trimethylphosphine for Alkene Hydrosilylation Reactions

In this paper, six silyl [PSiP] pincer cobalt(II) chlorides 1-6 [(2-Ph2PC6H4)2MeSiCo(Cl)(PMe3)] (1), [(2-Ph2PC6H4)2HSiCo(Cl)(PMe3)] (2), [(2-Ph2PC6H4)2PhSiCo(Cl)(PMe3)] (3), [(2-iPr2PC6H4)2HSiCo(Cl)(PMe3)] (4), [(2-iPr2PC6H4)2MeSiCo(Cl)(PMe3)] (5), and [(2-iPr2PC6H4)2PhSiCo(Cl)(PMe3)] (6)) were prepared from the corresponding [PSiP] pincer preligands (L1-L6), CoCl2 and PMe3 by Si-H bond activation. The catalytic activity of complexes 1-6 for alkene hyrdosilylation was studied. It was confirmed that complex 1 is the best catalyst with excellent regioselectivity among the six complexes. Using 1 as the catalyst, the catalytic reaction was completed within 1 h at 50 °C, predominantly affording Markovnikov products for aryl alkenes and anti-Markovnikov products for aliphatic alkene substrates. During the investigation of the catalytic mechanism, the Co(II) hydrides [(2-Ph2PC6H4)2MeSiCo(H)(PMe3)] (8) and [(2-iPr2PC6H4)2MeSiCo(H)(PMe3)] (9) were obtained from the stoichiometric reactions of complex 1 and 5 with NaBHEt3, respectively. Complexes 8 and 9 could also be obtained by the reactions of preligands L1 and L5 with Co(PMe3)4 via Si-H bond cleavage. More experiments corroborated that complex 8 is the real catalyst for this catalytic system. Under the same catalytic conditions as complex 1, using complex 8 as a catalyst, complete conversion of styrene was also achieved in 1 h, and the selectivity remained unchanged. Based on the experimental results, we propose a plausible mechanism for this catalytic reaction. The addition of B(C6F5)3 to catalyst 1 can reverse the selectivity of styrene hydrosilylation from the Markovnikov product as the main product (b/l = 99:1) to the anti-Markovnikov product as the main product (b/l = 40:60). Further study indicated that using the (CoCl2 + L1) system instead of complex 1, the selectivity was changed from Markovnikov to anti-Markovnikov product (b/l = 1:99.7). Therefore, the selectivity for the substrate styrene is influenced by the presence of a PMe3 ligand. The different selectivities may be caused by different active species. For the system of complex 1, a cobalt(II) hydride is the real catalyst, but for the (CoCl2 + L1) system, a cobalt(I) complex is proposed as active species. The molecular structures of Co(II) compounds 5 and 9 were resolved by single-crystal X-ray diffraction.

Photocatalytic Synthesis of Diarylmethyl Silanes via 1,6-Conjugate Addition of Silyl Radicals to p-Quinone Methides

A novel photocatalytic method for the preparation of diarylmethyl silanes was reported through silyl radicals addition strategy to p-QMs (p-quinone methides). This protocol could tolerate a variety of functional groups affording the corresponding silylation products with moderate to excellent yields. The resulting silylation products could be easily converted into a series of bioactive GPR40 agonists and useful p-QMs precursors for the synthesis of compounds possessing both quaternary carbon centers and silicon substituents through simple operation. A plausible mechanism of silyl radicals to p-QMs was proposed on the basis of experimental results and previous literature.

Synthesis of aryl cobalt and iron complexes and their catalytic activity on hydrosilylation of alkenes

Four aryl Co and Fe complexes, (F4C5N)CoCl(PMe3)3 (1), (F4C5N)Fe(PMe3)4 (2), (F5C6)CoCl(PMe3)3 (3) and (F4C5)FeCl(PMe3)3 (4), were synthesized from the reactions of 3-chloro-2,4,5,6-tetrafluoro-pyridine and chloropentafluorobenzene with Co(PMe3)4 and Fe(PMe3)4, respectively.